Designing for Fluctuating Loads

Problems for completely reversible loads are described in Chapter 10. The present chapter describes the fatigue strength due to fluctuating loads. For fluctuating loads, there exists a mean value of load Fm and variable value Fa. Dividing these loads by area of cross-section A, mean stress σm and variable stresses σa are calculated with the following relations:

When both the components of these stresses exist, these are handled by any one of the following theories, described in the following sections.

• • Soderberg line (Section 11.2.1)

• • Goodman lines line (Section 11.2.2 and 11.2.3)

• • Gerber Parabola (Section 11.2.4)

• • ASME elliptical curve (Section 11.2.5)

Safe and Unsafe Zones

Fatigue or endurance strength depends on both mean stress σm and variable stress amplitude σa. Fatigue diagram for fluctuating loads is shown in Figure 11.1 with mean stress on the X-axis and variable stress amplitude on the Y-axis.

A straight line joining tensile yield strength Syt on both the axes called yield line. It is shown dashed in Figure 11.1. If there is stress indicated by a point above the yield line, it will surely fail.

Load line OP is an inclined line starting from the origin on the fatigue diagram and slope as where Sm is the strength for mean stress and Sa is for variable stress. P is any general point indicating mean and variable stresses on this diagram.